The invention relates to a motor and magnetic compensation method thereof, and in particular to a brushless DC motor and magnetic compensation method providing a magnetic element to compensate a Hall sensor for magnetic bias interfered by an ambient magnetic pole.
Referring to FIG. 1, which is a schematic sectional view of a conventional brushless motor. In FIG. 1, a conventional brushless DC motor (m) includes a rotor 10 and a stator 20. The rotor 10 is a hollow and cylindrical rotor, with a closed end and an open end, includes a shaft 102 centrally disposed on the closed end of the rotor 10 and extended toward the stator 20, and a magnetic ring 103 disposed in the rotor 10 to enclose the stator 20.
The stator 20 includes a base 201, multiple silicon-steel sheets 202 stacked and disposed around the exterior of the seat 201 and wound by coils, a bearing 203 disposed in the base 201 to support the shaft 102 of the rotor 10, and a circuit board 204 disposed on the base 201 and located under the silicon-steel sheets 202. The circuit board 204 includes a control circuit (not shown) and a Hall sensor 205 for inducing the magnetic ring 103.
When the brushless DC motor (m) is actuated, the Hall sensor 205 on the circuit board 204 is coupled to the magnetic pole of the magnetic ring 103 of the rotor 10, and then a small voltage difference is output to the control circuit of the circuit board 204 according to magnetism of the magnetic ring 103. Based on the voltage difference output from the Hall sensor 205, the control circuit of the circuit board 204 determines whether the magnetic poles of the stator 20 are switched. Thus, the Hall sensor 205 coupled to the magnetic poles of the stator 20 drives the rotor 10.
If the distance between the Hall sensor 205 and the magnetic ring 103 is too great to output a regular voltage difference, i.e., a greater distance from the Hall sensor 205 to the magnetic ring 103 causes a relatively smaller magnetism induced by the Hall sensor 205. Further, if the Hall sensor 205 is interfered by the ambient magnetism, the accuracy of the output signal from the Hall sensor 205 is greatly affected. For example, if there are other magnetic elements disposed in the brushless DC motor (m), the accuracy of the output signal from the Hall sensor 205 is decreased due to ambient unnecessary magnetic influence.
Referring to FIG. 2A, which is a diagram showing the relationship between output voltage difference and phrase of magnetic pole for a Hall sensor of a conventional brushless motor interfered by ambient magnetism. In FIG. 2A, when the Hall sensor 205 is interfered by the ambient magnetic poles, irregular and asymmetric signals are produced between the N and S magnetic poles. Therefore, the magnitude of magnetism output from the stator 20 as determined by the control circuit of the circuit board 204 and the timing to switch the magnetic poles of the stator 20 are affected. Furthermore, the irregular signals between the N and S magnetic poles may damage the coupling between the rotor 10 and stator 20, resulting in a decelerating, vibrating or even locked rotor 10 during the operation.
Thus, it is an important issue that how to eliminate or reduce unnecessary magnetism interference to the Hall sensor in a motor.